1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for controlling beam current uniformity in the ion beam extraction region of an ion implanter.
2. Discussion of Related Art
Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. An ion implanter generally includes an ion source chamber which generates ions of a particular species, a series of beam line components to control the ion beam and a platen to secure the wafer that receives the ion beam. These components are housed in a vacuum environment to prevent contamination and dispersion of the ion beam. The beam line components may include a series of electrodes to extract the ions from the source chamber, a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer, and a corrector magnet to provide a ribbon beam which is directed to a wafer orthogonally with respect to the ion beam to implant the ions into the wafer substrate. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium are doped to create p-type regions in the substrate.
The series of electrodes are regulated at particular voltages with respect to ground to extract ions from the ion source chamber. For ribbon beams each of the electrodes include a slot having a particular length to extract the ions into a beam for downstream wafer implantation. When an ion beam is extracted, variations in beam related characteristics, such as contaminants, pressure, temperature, beam drift etc. influence beam uniformity. FIG. 1 illustrates a plot of beam position across the length of an elongated slot of an extraction electrode against beam current densities. As can be seen, the beam profile includes a plurality of variations in beam current densities within the range of ±2 mA/cm for positions −125 mm to approximately 75 mm. It is believed that these non-uniformities are the result of various electrode characteristics. Ideally, the current density of the beam components would be constant about point E. The disparity in uniformity along the length of the electrode slot compromises the uniformity of the beam at wafer implantation. Prior attempts to correct for these non-uniformities include providing a segmented electrode with conducting sections and insulating sections. The insulating sections are used to prevent interference between the electrode sections. Another attempt at correcting for these non-uniformities included providing a plurality of insulating rings surrounding a corresponding electrode segment. Actuators are used to displace only the insulating ring portions perpendicular to the electrode segments to control the power delivered to each electrode segment. However, these prior attempts have not sufficiently corrected the non-uniformities resulting from extraction of an ion beam from an ion source chamber. In addition, corrector magnet rods and poles may also be employed in ion implanters to provide beam uniformity tuning. However, this may require long tune times for certain desired beam profile features. Thus, there is a need to provide a beam profile pre-tuning method utilizing an electrode configuration which is capable of extracting an ion ribbon beam having a uniform beam profile for wafer implantation.